Working pressure rating for cast iron (C.I.) water mains Identify the typical maximum working pressure (approximate) to which C.I. pipes may be subjected in traditional practice.

Introduction / Context:
Pipe classes are chosen to withstand internal pressure plus surge allowances. Traditional cast iron mains have long been rated by nominal internal pressure capability tied to wall thickness/class.

Given Data / Assumptions:

• Typical historical classes for C.I. water mains in municipal distribution.
• Working (steady) pressure, not transient surge.

Concept / Approach:
Widely cited values for C.I. mains place the upper range of routine working pressures around 7 kg/cm^2 (approximately 0.7 MPa), with selection depending on diameter, class, and safety margins for transients.

Step-by-Step Solution:

Verification / Alternative check:
Historical specifications and catalogs list similar limits for standard C.I. classes; ductile iron extends capability beyond these values as needed.

Why Other Options Are Wrong:

• 3 or 5 kg/cm^2 may be too conservative for many trunk mains.
• 10 or 12 kg/cm^2 exceed typical C.I. classes used widely without special design.

Common Pitfalls:
Not accounting for surge; even at 7 kg/cm^2 steady pressure, surge control (air valves, surge tanks) is vital.

Final Answer:
7 kg/cm^2